The present invention is directed to a gap stitch mechanism for a sewing machine. The sewing machine is provided with a needle shaft which is oscillatingly driven by a crank assembly.
Gap stitch mechanisms are well known in the art. They include a cyclically controllable device which prohibits stitch formation at intervals. It is thus possible to form stitches several times the length of the forward feeding movement of the material. A known device of this type consists, for example, in stopping the needle shaft at intervals in its top dead center position, either by interrupting the needle shaft drive, as is shown, for example, in the Swiss Pat. No. 606,571, or by releasing the coupling at intervals between the feeder of the needle shaft drive and the needle shaft, as described, for example, in German Pat. No. 1,685,775. These gap stitch mechanisms usually have the disadvantage of creating excessive noise and causing rapid wear.
Accordingly, an object of the present invention is to provide a gap stitch mechanism which eliminates the need to connect and disconnect the needle shaft to prevent the formation of stitches, which leads to the disadvantages mentioned above. To this end, the present invention is characterized by the fact that means are provided for utilizing a crank assembly which reduces the crank radius, which in turn determines the needle shaft stroke, at predetermined intervals to the extent that it is impossible for the loop catcher to catch the needle thread. This solution eliminates the need for any additional interrupted needle shaft movement, i.e. the needle shaft operation continues at its normal operating rate thus, connecting and disconnecting the needle shaft and the crank mechanism are eliminated, as well as all noise and wear unavoidably associated with such processes.
It is customary to provide an arm shaft crank supported by the arm shaft of the device in the crank mechanism of the needle shaft drive, said arm shaft crank supporting a drive crank coupled with the needle shaft. The crank arm of this drive crank determines the crank radius which in turn determines the needle shaft stroke. One solution to the object according to the present invention comprises changing the crank radius by turning the drive crank in a bearing bore hole of the arm shaft crank. In an especially advantageous feature, an adjusting crank supporting the drive crank is turned in a bearing bore hole of the arm shaft crank, e.g. by way of an adjusting lever that is securely attached thereto. The adjusting lever works together with cams that can be chosen and moved into operating position. Since all drive members between the arm shaft and the needle shaft maintain their continuing movement pattern regardless of whether the drive has a full or abbreviated stroke, there is neither a large amount of noise generated, nor is there aggrevated wear, as would be the case without utilizing the gap stitch mechanism.